Acute kidney injury (AKI) is a serious and deadly disease process affecting 5-10% of all hospitalized patients. The mortality rate in these cases often exceeds 50%. AKI is independently associated with increased mortality rates in several clinical situations, including subsequent to administration of radio contrast dye and cardiovascular surgery. It is often multi-factorial in etiology, especially in critically ill patients. The relative importance of individual factors depends upon the underlying pathology and patient co-morbidities.
Recent data demonstrate an alarming increase in the total number of cases of AKI. Utilizing patient claims in the Medicare 5% sample from 1992-2001, Xue et al (J Am Soc Nephrol 17:1135-1142 2006) have shown that during this time period, the incidence of AKI increased approximately 11.6% per year from 23.6 cases per 1,000 discharges in 1992 to 63.3 cases per 1,000 patients in 2001.
In a recent study, Hsu et al (Hsu, et al., “Community-Based Incidence of Acute Renal Failure,” Kidney Int. 2007; 72(2):208-12.) quantified the incidence of non-dialysis and dialysis AKI among members of a large integrated health care delivery system. Between 1996 and 2003, the incidence of non-dialysis-requiring AKI increased from 323 to 522 while the incidence of dialysis-requiring AKI increased from 20 to 30 per 100,000 person years. Furthermore, hospital death rates were much higher in patients with AKI than in non-AKI discharges. Patients without AKI had a 4.6% in-hospital death rate while those with primary AKI and secondary AKI had rates of 15.2 and 32.6%, respectively. Death within 90 days after hospital admission was 13.1% in discharges without AKI, 34.5% and 48.6% of patients with primary and secondary AKI, respectively. In this large study, the probability of developing end stage renal disease was 18.8% in patients with acute kidney injury as a principle diagnosis and 10.1% in patients with acute renal failure as a secondary diagnostic code. Finally, using the data collected, it was calculated that at least 22.4% of the end stage renal disease (ESRD) cases in the United States come from Medicare beneficiaries who had hospital acquired AKI.
These data are in agreement with observations made by Dr. Paul Eggers, director of epidemiology NIDDK, indicating a rapid increase in the percentage and absolute number of hospitalized patients with AKI as a primary or secondary diagnosis and in patients with chronic kidney disease (CKD) progressing onto ESRD having had AKI as a hospital diagnosis.
In another study (Uchino, et al., “An Assessment of the RIFLE Criteria for Acute Renal Failure in Hospitalized Patients,” Crit. Care Med. 2006; 34(7):1913-7.) the incidence and outcomes of 20,126 hospitalized patients was determined in a retrospective single-center study. Of these patients 14.7% required ICU admission, 18% had AKI, and mortality correlated with the extent of kidney injury. Finally, in a multi-center retrospective ICU study AKI occurred in 67% of admissions and again the overall prognosis correlated with the severity of AKI.
Clearly, the prevalence of AKI in hospitalized patients is increasing at an alarming rate. The severity of injury determines hospital outcomes, and AKI accelerates the development of chronic kidney disease and progression of CKD to ESRD.
It is believed that glomerular filtration rate GFR is the most relevant metric for determining the extent of AKI and progression of CKD. Reductions in the GFR secondary to kidney injury, either acute or chronic, are accompanied by increases in blood urea nitrogen (BUN) and serum creatinine levels. Currently, either serum creatinine or an equation based on the serum creatinine is used to determine a patient's estimated GFR (eGFR). Unfortunately, these two approaches are not reliable over the full range of GFR, and neither can be used in AKI, since both muscle mass (creatinine is a breakdown product of creatine, which is an important part of muscle) and GFR determine a patient's serum creatinine level.
Using serum creatinine as an indicator of GFR is highly patient specific. For instance, a serum creatinine of 1.0 mg/dl is indicative of a normal GFR (100 ml/min) in a 70 Kg (154 lb) male with normal muscle mass. However, in a 50 Kg (110 lb) male with moderate muscle wasting, a serum creatinine of 1.0 mg/dl is seen even though his GFR is only 50 ml/min. Formulas derived from large population studies have been developed to factor in patient weight, age, sex and race. However, even these formulas are inaccurate and often misleading in estimating GFR below 20 or above 60 ml/min. Therefore, this is another reason they cannot be used in the setting of AKI.
Recent data indicate that even very small changes in kidney function, as determined by small total equilibrium elevations in serum creatinine, previously felt to be clinically insignificant, are now known to predict an increased mortality rate. Several recent publications have utilized the Risk, Injury, Failure, Loss and ESRD criteria (often called “RIFLE” criteria) to stratify patients into apparent levels of injury based on the maximum serum creatinine obtained and the need for dialysis. Data collected for mortality, length of hospital stay (LOS), LOS of ICU stay, hospital costs, and the need for renal replacement therapy related to the highest stage achieved in this stratification system. These data indicate that the severity or extent of kidney injury in AKI is an important prognostic indicator of a patient's outcome. Furthermore, early changes in organ function predict survival in severe sepsis. 100121 Serum creatinine determinations as a measure of GFR may also be severely limiting because of the time it takes to reach equilibrium values required for an accurate conversion. Patients with acute renal failure develop an abrupt decline of their GFR; however, the magnitude of this decline is only apparent after several days of equilibration if determined by a rising serum creatinine. For instance, if a patient was to lose 95% of his GFR secondary to AKI, the GFR would decrease from 100 to 5 ml/min rapidly, but the serum creatinine would only rise by 1 mg/dl/day. This slow rise in serum creatinine limits the physician's ability to diagnose the injury for 12-24 hours after the event, and it is also not possible to determine the extent of injury for days. This has markedly limited the ability to conduct a therapeutic trial in AKI. Since the extent of the decline in GFR, or eventual plateau in serum creatinine, correlates with morbidity, mortality and recovery potential, the ability to accurately determine GFR in patients with acute kidney injury is of great clinical importance for rapid diagnosis, stratification and timely treatment.
It is widely held that beginning therapy after 12-24 hours of AKI may limit the success rate of any potential therapeutic agent. Therefore, a search for a biomarker of kidney injury has intensified and is now considered by many experts to be the highest priority in the field of AKI. Potential molecules include NGAL, KIM-1, IL-18, and several others. Any one biomarker, or probably a combination of biomarkers, will serve as structural markers of injury. However, improvements sought utilizing these structural biomarkers may not be significant because they were developed using population results that may not apply to an individual.
Collection of a 24 hour urine and invasive techniques exist to accurately determine a patient's GFR, but these are cumbersome, error prone, expensive, time consuming, or expose the patient to radiation or radio contrast media. Also, there is no rapid and accurate measurement technique that can determine GFR reliably in patients with acute kidney injury when the serum creatinine is rising.
The present invention is provided to solve the problems discussed above and other problems, and to provide advantages and aspects not provided by prior diagnostic techniques. A full discussion of the features and advantages of the present invention is deferred to the following detailed description, which proceeds with reference to the accompanying drawings.